Novel ring waveguide device in a 2D photonic crystal slab - Transmittance simulated by finit-difference time-domain analysis

We propose a novel ring waveguide device in a two-dimensional (2D) photonic crystal (PhC) slab with a hexagonal lattice of air holes in a semiconductor (Si/GaAs). The device consists of a single line-defect ring waveguide and other parts. We simulated the propagation of a 1 ps pulse in the device. We analyzed the device using the 2D finite-difference time-domain (FDTD) method. The 1 ps pulse is transmitted well through sharp 60 degree bends in the ring waveguide. We designed and examined four-port directional couplers (DC) as input/output (I/O) components for the ring device. Coupling between the I/O waveguides and the ring waveguide in the device can be achieved by the four-port DC. We adopted a defect pattern formed with a straight waveguide and a waveguide with bends as the DC structure. This pattern realizes the coupling length based on the coupled mode theory. Our DC with the new multistep-like waveguides improved both the directivity and coupling factor across most of the frequency band where there are practical guided-wave modes in the photonic band gap (PBG). The propagation loss of a 1 ps pulse in the ring waveguide is low, so the transmission property of the ring device depends mainly on the I/O parts. In this study, we confirmed that the ring device would be sufficient for such applications as delay lines.